As scientists rule out competing theories, the case for life on the red planet grows stronger.

Even though the Mars rovers and other probes sent to the red planet have yet to stumble onto any alien life forms, scientists believe there's a very good chance something is alive down there.

Bolstering their case this week is the report that methane levels in the atmosphere of Mars cannot be caused by meteorites burning up during entry — a long-held explanation. The gas is constantly being replenished from an unknown source, but a new study showed "that the volumes of methane that could be released by the meteorites entering Mars's atmosphere are too low to maintain the current atmospheric levels of methane."

The scientists also long ago ruled out methane released by volcanic activity. So what might be causing it?

The most hopeful explanation is that there are microorganisms living in the Martian soil that are creating methane gas as a byproduct of their metabolic processes. Professor Mark Sephton of the Department of Earth Science and Engineering at Imperial College London and co-author of the report, told Science Daily: "This work is a big step forward. As Sherlock Holmes said, eliminate all other factors and the one that remains must be the truth. The list of possible sources of methane gas is getting smaller and excitingly, extraterrestrial life still remains an option. Ultimately the final test may have to be on Mars."

The less exciting remaining theory is that methane is the byproduct of reactions between volcanic rock and water.

Scientists eager to get more information to back up either explanation will have to wait a bit longer for some serious data. In 2018, NASA and the European Space Agency are planning a joint mission to the red planet in 2018 to discover the source of the methane.